There exist in today's market imaging devices capable of reading documents such as ID cards, drivers license, business cards and the like. In imaging devices that include an imaging sensor array, based on technologies such as a CCD or CMOS, it is required to illuminate the imaged document. It is desired that the illumination will of high intensity and substantially uniform, in order to obtain a substantially true image of the imaged document.
The document is typical place on the surface of a substantially flat and substantially transparent panel, typically, with no limitation, made of glass (herein, also referred to as “glass-window”). But the glass surface is also a reflective surface, returning a portion of the incident light rays striking the transparent glass surface. Thereby, the image frame acquired from the imaged document is distorted. An internal light source that directly illuminate the glass-window bring are reflected from the glass surface, thereby forming “hot spots” that distort the uniformity of the illumination of the imaged document, and thereby causing the image of the imaged document not to be a substantially true image of the imaged document.
Reference is now made to FIG. 1a (Prior art) and to FIG. 1b (Prior art). FIG. 1b depicts a pair of hot spots 40 caused by direct illumination of the imaged document disposed on the external surface of a glass-window 20, as viewed from inside the camera housing. FIG. 1a illustrates how hot spots 40 are formed by direct illumination of the imaged document 10 disposed on the external surface of a glass-window 20. When imaging document 10 with a camera 50, document 10 needs to be illuminated. In a conventional illumination method, as shown in FIG. 1a, illumination sources 30a and 30b are positions to directly illuminate glass-window 20. Such configuration enables the return of the majority of light to the lens of camera 50, but gives rise to a hot spot 40, where the light source (30) itself is imaged by camera 50.
US patent application 20080285094, by Hatzav et al., provides a configuration method of the illumination system that reduces the hot spot problem, by disposing the light sources outside the field of view (FOV) of the camera. Reference is now made to FIG. 2 (Prior art), illustrating a pair of light sources 30 that are disposed outside the conventionally preconfigured FOV 60 of camera 50, which FOV 60 of camera 50 is denoted by virtual lines 62. Furthermore, the inner walls 55 and 75 of inner space 70 of the prior art document imaging device are white to further enhance the uniformity of the illumination. However, back-light (indirect) illumination brings about loss of light intensity and requires a high intensity light source. Furthermore, hot spots of lower intensity, with respect to direct illumination, still appeared in the image formed.
Thus there is a need for and it would be advantageous to have a camera based imaging device that has an illumination system that provides uniform illumination with no reflections, hot spot effects, and other illumination artifacts, as well as avoiding substantial loss of illumination intensity.